The curing rate of radiation curable silicone compositions for coating applications or adhesive applications is a very important factor in determining the effectiveness of the composition. The problem with free radical curing systems is their inhibition in the presence of oxygen and thus these systems require costly inert gas facilities to ensure a satisfactory cure rate. On the other hand, cationic curing systems have no oxygen inhibition problems but have the problem of moisture sensitivity.
Efforts to overcome these problems have been disclosed in the art. Furthermore, radiation curable compositions which contain vinyl ether functional organosilicon compounds have also been described in the art. For example, Crivello in U.S. Pat. No. 4,617,238 discloses a photopolymerizable composition comprising (a) an organopolysiloxane having at least one Si-bonded vinyloxy functional group of the formula H.sub.2 C.dbd.CH--O--G--, where G is alkylene (such as propylene) or alkylene interrupted by at least one divalent heteroradical selected from --O--, divalent phenylene, or substituted divalent phenylene, or combination of such heteroradicals, and (b) an onium salt catalyst. The '238 patent also describes a method wherein the vinyl ether group is introduced into the organopolysiloxane by addition (hydrosilylation) of compounds with an allyl and a vinyl ether group to an SiH group of the organopolysiloxane in the presence of a platinum catalyst. In the method of the '238 patent, only the allyl group is added to the SiH group while the vinyl ether group is preserved and thus only one vinyl ether group for each SiH group can be incorporated into the siloxane molecule at any given time.
European Patent Publication No. 0462389 teaches thermosetting organopolysiloxanes with oxyalkylene vinyl ether groups bonded by SiOC groups and the vinyl groups may be substituted by alkyl radicals. EPO'389 also teaches a method for the preparation of these compounds and their application as photochemically thermosetting polysiloxanes in encapsulating compounds, as non-stick coating compounds for flat carriers or as modified additives in compounds which can be thermoset radically, cationically or by UV or electron radiation.
Brown et al., in U.S. Pat. No. 5,270,423 discloses organosilicon compounds with a siloxane portion of the general formula --OR'OCH.dbd.CHR" linked via an SiOC bond wherein R' is a divalent hydrocarbon radical and R" is hydrogen or an alkyl radical which are useful in radiation curable compositions, in which they are mixed with an initiator. The compositions are particularly useful in UV radiation curable coatings.
Glover et al. in U.S. Pat. No. 5,594,042 discloses radiation curable compositions comprising vinyl ether functional siloxanes and aromatic iodonium salt or aromatic sulfonium salt photoinitiators which cure upon exposure to ultraviolet or electron beam radiation. The vinyl ether groups are linked to the silicon atom on the siloxane through an SiOC bond and the photoinitiators are disclosed as being preferably either diaryliodonium salts of sulfonic acids or triarylsulfonium salts of sulfonic acids.
Bujanowski et al. in U.S. Pat. No. 5,629,095 discloses vinyl ether functional siloxane resins, radiation curable coating compositions comprising a vinyl ether functional siloxane resin and a photocleavable acid, and a coated article obtained by applying the radiation curable coating composition to a substrate and then exposing the coating to radiation in an amount sufficient to cure the coating. In the '095 patent, the vinyl ether group in the siloxane resin is attached to the silicone atom through an SiOC bond.
In contrast to the above described compositions, the radiation curable silicone compositions of this invention have a good curing rate in moist air without requiring the protection afforded by an inert gas facility.